The present invention provides a process for preparing a composition containing gold and/or silver particles and an amorpous titanium/silicon mixed oxide, compositions which can be prepared in accordance with this process and their use as catalysts for the partial oxidation of hydrocarbons.
The sol-gel process is known [L. C. Klein, Ann. Rev. Mar. Sci., 15 (1985) 227; S. J. Teichner, G. A. Nicolaon, M. A. Vicarini and G. E. E. Garses, Adv. Colloid Interface Sci., 5 (1976) 245]. However, this process has not been used hitherto to prepare compositions which contain gold and/or silver particles and an amorphous titanium/silicon mixed oxide, as a method for preparing catalysts for direct oxidation with molecular oxygen as a reducing agent, because the suitability of compositions prepared therefrom for the catalytic oxidation of hydrocarbons has not been disclosed.
Crystalline titanium silicalite catalysts are known.
U.S. Pat. No. 4,833,260 describes crystalline titanium silicalite catalysts which enable the effective epoxidation of olefins with the oxidising agent hydrogen peroxide in the liquid phase. In silicalites, a small proportion of the silicon in the lattice has been replaced by titanium (U.S. Pat. No. 4,410,501).
On platinum metal-containing titanium silicalites, propene oxidation proceeds with small yields (about 1-2%) and propene oxide selectivities of 60-70% in the liquid phase due to in situ hydrogen peroxide production using a gas mixture consisting of molecular oxygen and molecular hydrogen (JP-A 92/352771, WO 97/47386, WO 96/023 023). Hydrogenations which occur as a secondary reaction lead to large amounts of propane as a secondary product and the fact that this is a liquid phase reaction in which the epoxide being produced accumulates in the liquid phase means that this process is of little interest for industrial use.
U.S. Pat. No. 5 623 090 describes a gas phase direct oxidation of propene to propene oxide with relatively low propene conversions low (0.5-1% propene conversion, with respect to a 10% strength propene feed concentration) but with propene oxide selectivities of  greater than 90% with oxygen as the oxidising agent. This is a gold/titanium dioxide catalysed gas phase oxidation with molecular oxygen in the presence of hydrogen at temperatures of 40-70xc2x0 C. The catalyst which is used is commercially available crystalline titanium dioxide with a very high proportion of the anatase modification (P25, Degussa; 70% anatase and 30% rutile), which is coated with nano-scale gold partides using a deposition-precipitation method. This process has the large disadvantage, in addition to relatively low propene conversions, that the disclosed catalysts deactivate greatly with time. Typical half lives at atmospheric pressure and 50xc2x0 C. are 30-150 minutes. Increasing the temperature and/or pressure in order to raise the conversion shortens the half lives even further.
In another embodiment, with the same reactant gases, catalysts are used in which gold particles are applied to a support consisting of finely dispersed titanium centres on a silicon dioxide matrix (analogous to the shell variant [U.S. Pat. No. 3,923,843], a heterogeneous titanium and silicon-containing material is used, which is prepared by impregnating SiO2 with titanium precursors in solution) (WO 9800415 A1; WO 9800414 A1; EP 0 827 779 A1). All these catalysts, which are obtained from materials by impregnation of the purely inorganic silica surface with titanium precursors in solution followed by coating with gold by deposition-precipitation and subsequent calcination in an atmosphere of air, exhibit relatively low propene conversion and deactivate rapidly (typical half lives are 10-50 h) and therefore cannot readily be used in industrial scale plant.
WO-98/00413 discloses catalysts in which gold particles are applied to inorganic, microporous, silicates with a crystalline structure with defined pore structures (e.g. TS-1, TS-2, Ti-zeolites such as Ti-beta, Ti-ZSM-48 or titanium-containing, mesoporous molecular sieves such as e.g. Ti-MCM-41 or Ti-HMS). Although all these purely inorganic gold/silicate or gold/zeolite catalysts exhibit good selectivities during partial oxidation, the conversions of hydrocarbons, and in particular the catalyst lifetimes, are inadequate for application in the chemical industry.
The methods described for preparing the catalysts are highly unsatisfactory with respect to catalyst activity and lifetime. Industrial processes which use low activity catalysts require very large reactors. Low catalyst lifetimes restrict production output during the regeneration phases or require duplicated, cost-intensive production routes.
Thus the development of a process to prepare catalysts with which excellent selectivities and high activities can be achieved with industrially relevant lifetimes is required.
Furthermore there is a requirement for a domain-free structure in the catalysts.
There is also the object of reducing the disadvantages in the process according to the prior art.
Another object of the present invention is to provide a technologically simple catalytic gas phase process for the selective oxidation of hydrocarbons with a gaseous oxidising agent on economically viable solid catalysts which leads to high yields and low costs with very high selectivities and industrially relevant catalyst lifetimes.
These objects are achieved according to the invention by the provision of a process for preparing a supported composition which contains gold and/or silver particles and an amorphous titanium/silicon mixed oxide, characterised in that the titanium/silicon mixed oxide is prepared by a sol-gel process and that organic/inorganic sol-gel hybrid systems are preferably prepared.